The invention relates to a system which may be used to measure the noise levels of laser devices, including continuous wave and pulsed lasers and, in particular, semiconductor laser diode devices such as those used in telecommunications. The system may be used to measure relative intensity noise (RIN) levels. RIN measurements are of particular importance in the field of fibre optic communications.
Spurious, random laser omissions cause intensity fluctuations in the output from laser sources. These fluctuations form the noise floor which varies with frequency and peaks at the relaxation resonance of the laser. The location of the relaxation resonance, intensity noise peak, is directly related to the maximum modulation rate of the laser. Interaction between the optical field in the laser and the injected-electron density, due to the bias current, causes the relaxation resonance to vary as a function of the bias current.
An important parameter in evaluating both laser and system performance for broadband digital and analogue systems is the relative intensity noise (RIN). RIN is the ratio of the mean-squared-intensity-fluctuation spectral density of the optical power output to the square of the average optical power. Some known measurement systems for measuring RIN are based around the use of a photodetector diode, a high gain amplifier and an electrical spectrum analyser. The optical laser energy is transferred to electrical energy by the photodetector diode and is amplified to enable low level intensity noise to be measured by the spectrum analyser.
Existing systems for measuring RIN of a laser are described in the following references; U.S. Pat. No. 5,534,996 and M. Puleo, "Ultra-high sensitivity technique for characterisation of laser diode intensity noise in the GHz range", CSELT Technical Reports, vol. 17, no. 3, June 1989, Italy pp 199-200.
One of the disadvantages of existing systems is that the frequency response and linearity of the photodetector, amplifier and spectrum analyser must be well characterised to obtain accurate measurements across a useful, wide frequency range, typically between 20 MHz and 20 GHz. Furthermore, factors such as mismatch losses, detector capacitance and spectrum analyser amplitude errors also need to be accounted for and this makes system calibration complex and time consuming.
JP A 02 189 474 describes an apparatus for measuring the noise of an optical receiver, as opposed to an optical source, and is general background to the present invention.
The present invention relates to a system and method for measuring noise levels of laser devices. In particular, the system may be used to measure relative intensity noise levels for semiconductor laser diodes such as those used in telecommunications. The system design is such that system calibration can be calculated simply and frequently, reducing systematic errors and calibration costs. Using this system, RIN measurements with an accuracy of less than 1 dB may be achieved across a frequency range of between 10-20 MHz and 20 GHz.